1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an efficient spatial modulation scheme for achieving diversity gain and high transmission rate.
2. Discussion of the Related Art
As an example of a wireless communication system to which the present invention is applicable, a 3rd generation partnership project (3GPP) long term evolution (LTE) communication system will be schematically described.
FIG. 1 is a schematic diagram showing a network structure of an evolved universal mobile telecommunications system (E-UMTS) as an example of a wireless communication system. The E-UMTS is an evolved form of the legacy UMTS and has been standardized in the 3GPP. In general, the E-UMTS is also called an LTE system. For details of the technical specification of the UMTS and the E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.
Referring to FIG. 1, the E-UMTS includes a user equipment (UE), an evolved node B (eNode B or eNB), and an access gateway (AG) which is located at an end of an evolved UMTS terrestrial radio access network (E-UTRAN) and connected to an external network. The eNB may simultaneously transmit multiple data streams for a broadcast service, a multicast service and/or a unicast service.
One or more cells may exist per eNB. The cell is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink (DL) or uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths. The eNB controls data transmission or reception to and from a plurality of UEs. The eNB transmits DL scheduling information of DL data to a corresponding UE so as to inform the UE of a time/frequency domain in which the DL data is supposed to be transmitted, coding, a data size, and hybrid automatic repeat and request (HARQ)-related information. In addition, the eNB transmits UL scheduling information of UL data to a corresponding UE so as to inform the UE of a time/frequency domain which may be used by the UE, coding, a data size, and HARQ-related information. An interface for transmitting user traffic or control traffic may be used between eNBs. A core network (CN) may include the AG and a network node or the like for user registration of UEs. The AG manages the mobility of a UE on a tracking area (TA) basis. One TA includes a plurality of cells.
In order to improve performance of the related art LTE communication system mentioned in the above description, ongoing discussions are made on 5G communication technology. And, the 5G communication system is expected to use spatial modulation scheme based on massive MIMO technology.
FIG. 2 is a diagram showing a difference between spatial multiplexing and spatial modulation.
FIG. 2 (a) is a diagram for explaining the spatial multiplexing scheme. According to spatial multiplexing scheme, different signals (S1 and S2) are transmitted via different transmission antennas. On the other hand, FIG. 2 (b) is a diagram for explaining the spatial modulation scheme. According to spatial modulation scheme, S1 is transmitted via antenna 0 or 1, and selection of antenna 0/1 represents S2. That is, S2 can be represent not based on the signals transmitted via each antenna, but based on selection of antennas for transmission.
So, spatial modulation (SM) can be referred to as a single-RF multiple-antenna transmission technique. The smaller RF-chain number and low detection complexity at the receiver of spatial modulation make it an energy-efficient modulation method for the massive MIMO system. According to Massive MIMO scheme to be employed to 5G standardization technology, the targeted MIMO dimension may amount up to hundreds of antennas and the transmitter and receiver.
However, the above explained spatial modulation has a problem in that it may suffer antenna specific error. For example, when the channel of antenna 0 is poor in the example of FIG. 2 (b), the transmission of S1 via antenna 0 might fail. So, the spatial modulation has to be modified to have spatial diversity gain.
Further, the transmission rate of spatial modulation is lower than spatial multiplexing scheme. For example, when there are Nt transmission antennas, and one symbol (S1) represents M information, the spatial multiplexing scheme can convey Nt log2(M) bits for one transmission. On the other hand, for the same environment, spatial modulation scheme can convey log2(Nt)+log2(M) bit for one transmission.